1. Field of the Invention
The present invention relates to an optical head device employed in for example an optical disk device.
2. Description of the Background Art
An optical disk device records and/or reproduces information by bringing an optical head device near to an optical recording medium. Generally, an optical head device comprises a light source, an objective lens mechanism, a photo detector and an optical system. In the optical head device, a light beam emitted from the light source is converged on the optical recording medium by means of an objective lens or the like. The converged light beam is reflected from the optical recording medium. The reflected light is led by the optical system to the photo detector. Then the optical head device carries out focusing and tracking for recording or reproducing information.
FIG. 1 schematically shows a structure of a general optical head device. Referring to the figure, the device comprises a fixed optical portion A and an objective lens driving mechanism B. An optical recording medium C is positioned below the objective lens.
The operation of the device shown in the figure will be described in the following. A light beam emitted from a semiconductor laser 1 is turned into parallel beam flux by a collimator lens 2. The parallel beam flux passes through a beam splitter 3 to enter the objective lens 4. The objective lens 4 converges the parallel beam flux on the optical recording medium C. The position of convergence is adjusted by driving the objective lens 4 by a driving mechanism, not shown. The optical recording medium C comprises a transparent plate 5, tracks 6 and guide tracks 6'. The alternating tracks 6 and guide tracks 6' are formed of a thin film of an amorphous alloy of rare earth or a transition metal constitute a recording medium. The light beam is controlled so as to converge on a track 6 by means of the objective lens 4. The light beam reflected from the optical recording medium C passes through the objective lens 4 to enter the fixed optical portion A. The light beam from the objective lens 4 is reflected by the beam splitter 3. The reflected light passes through a convergent lens 8 to enter a cylindrical lens 9. The light beam passed through the cylindrical lens 9 is turned into beam flux having astigmatism so as to converge on a photo detector 10 whose dividing line is inclined by 45.degree. with respect to the generator of the cylindrical lens 9. An output from the photo detector 10 is processed by circuit means, not shown, and then it is converted into a focusing error signal. A focusing mechanism of the objective lens is controlled by the focusing error signal.
The principle of detecting a focusing error in the above described optical head device will be describe with reference to FIGS. 2A to 2F. FIGS. 2A, 2C and 2E show optical paths of the light beam reflected from the optical recording medium C passing through the objective lens 4, the convergent lens 8 and the cylindrical lens 9 to reach the photo detector 10. FIGS. 2B, 2D and 2F show the light beam irradiating a photo sensitive portion 20 of the photo detector 10. As shown in the figures, the photo sensitive portion 20 comprises four elements 10a, 10b, 10c and 10d. The boundaries of the four elements are two lines intersecting with each other perpendicularly. The boundaries are inclined with respect to the generator of the cylindrical lens 9 shown by an arrow in the figure. The center of the photo sensitive portion, that is, the intersecting point of the boundaries, is positioned at midway between two focusing points generated by astigmatism, when the light beam converges on a track 6 of the optical recording medium C as shown in FIG. 2A.
FIGS. 2A and 2B show the light beam converged on a track 6. FIGS. 2C and 2D show the light beam converged in front of the track 6. FIGS. 2E and 2F show the light beam converged behind the track 6.
The focusing error signal (FES) is represented as the following equation (1), where Sa, Sb, Sc and Sd represent light intensities irradiating the elements 10a, 10b, 10c and 10d of the photo sensitive portion 20, respectively. EQU FES=(Sa+Sc)-(Sb+Sd) (1)
When the point of focus is on the track 6 as shown in FIG. 2A, a spot on the photo sensitive portion 20 is circular, and the light intensities irradiating the elements are the same, so that the signal FES provided in accordance with the above equation is 0. When the point of focus is in front of the track 6 as shown in FIG. 2C, the spot on the photo sensitive portion 20 is an ellipse having its major axis parallel to the generator (shown by an arrow) of the cylindrical lens. In this case, the light intensities Sb and Sd are higher than Sa and Sc. Therefore, the signal FES has a negative value. When the point of focus is behind the track 6 as shown in FIG. 2E, the spot on the photo sensitive portion 20 is an ellipse having its major axis vertical to the generator of the cylindrical lens 9. Then the light intensities Sa and Sc are higher than Sb and Sd, and the signal FES has a positive value. Based on the principle described above, the direction of defocus depends on whether the signal FES is positive or negative, and the amount of defocus depends on the level of the signal FES. Accordingly, the objective lens driving mechanism moves the objective lens to an appropriate position in accordance with the signal FES to converge the light beam on the optical recording medium.
Now, description will be given of a crosstalk in the focusing error signal caused when the position of the track is off the position of the light beam converged on the optical recording medium C.
Let us assume that there is no aberration in the optical system comprising a collimator lens, a beam splitter, a convergent lens, a cylindrical lens and an objective lens. FIGS. 3A to 3F show this ideal state. FIGS. 3A, 3C and 3E show positional relation between a track 6 of the optical recording medium C and a spot 11 of a light beam irradiating the optical recording medium C. FIGS. 3B, 3D and 3F show spot shapes of the light beam irradiating the photo sensitive portion 20 of the photo detector 10. If the center of the spot 11 is off the track 6 as shown in FIGS. 3A and 3E, the shape of the spot on the photo sensitive portion 20 is as shown in FIGS. 3B and 3F, respectively. If the track 6 is positioned at the center of the spot 11 as shown in FIG. 3C, the shape of the spot of the light beam irradiating the photo sensitive portion 20 is circular as shown in FIG. 3D.
As shown in FIGS. 3A to 3F, the horizontal boundary X--X of the photo sensitive portion 20 is parallel to the longitudinal direction of the track 6, while the vertical boundary Y--Y is vertical to the longitudinal direction of the track 6. The photo sensitive portion 20 is arranged such that the optical axis of the light beam passes through the center thereof. When the track 6 is positioned at the center of the spot 11, the spot will be circular as shown in FIG. 3D. Meanwhile, if the position of the track 6 is off as shown in FIGS. 3B and 3F, distribution of light intensity irradiating the photo sensitive portion 20 of the detector changes dependent on how much the track is off. If the optical system has no aberration, the distribution of light intensity is symmetrical about the boundary Y--Y. Therefore, even if the position of the track is off, the value of the focusing error calculated in accordance with the equation (1) is 0. Namely, imbalance of the distribution of the light intensity is offset, and the focusing error signal is not changed dependent on the deviation.
Next, let us assume that the optical system has the aberration. FIGS. 4A to 4F show deviation of the track and the shapes of the spot irradiating the photo sensitive portion of the photo detector when the optical system has the aberration. The three cases shown in FIGS. 4A to 4F correspond to the three cases shown in FIGS. 3A to 3F, respectively.
In the ideal case where the optical system has no aberration, the light spot irradiating the photo sensitive portion 20 of the photo detector 10 is symmetrical about the boundary Y--Y as shown in FIGS. 3A to 3F. If the optical system has the aberration, the light spot is distorted as shown in FIGS. 4B, 4D and 4F. The distortion is especially conspicuous when the direction of the aberration is inclined by about 45.degree. to the longitudinal direction of the track 6. The distortion is not noticed when the direction of aberration is vertical or parallel to the longitudinal direction of the track 6. When the center of the light spot 11 is off the track 6, the shape of the light spot on the photo sensitive portion 20 is as shown in FIGS. 4B and 4F, respectively. The shapes of the light spot represented by the hatched portions in FIGS. 4B and 4F are both asymmetrical about the boundary Y--Y. In the case shown in FIGS. 4A and 4B, the focusing error signal is recognized as a positive value, while in the case of FIGS. 4E and 4F, it is recognized as a negative value. Therefore, it is proved that erroneous signals are generated due to the aberration. FIG. 5 shows a waveform of a focusing error signal generated when the light spot 11 crosses the track 6. In FIG. 5, the abscissa represents distance between the objective lens 4 and optical recording medium C, while the ordinate represents the level of the focusing error signal. If the optical system has an aberration, an erroneous signal is generated due to the deviation between the track 6 and the light spot 11, so that the waveform of the focusing error signal fluctuates. Especially, there is considerable fluctuation near the point of in-focus. In an ideal state where the optical system has no aberration, the waveform of the focusing error signal is a smooth curve. The aberration existing in the optical system makes unstable the control of focusing, and may prevents normal recording and reproducing.
Japanese patent Laying Open No. 63-244416 discloses an optical information detecting device comprising an optical system having a flat glass plate for reducing influence of aberration. FIG. 6 shows one embodiment of the optical information detecting device. In this device, a flat plate 12 formed of glass having both surfaces flat is arranged between a collimator lens 2 and a semiconductor laser 1 as a light source. The flat plate 12 can be rotated about a optical axis 13. Other components of the device are the same as those shown in FIG. 1. When a flat plate having the thickness of t and a reflective index of n is arranged inclined by .theta. in a light beam having an angular aperture of .alpha., generated astigmatism w is represented as: ##EQU1##
This device eliminates the astigmatism remaining in the optical system by utilizing the astigmatism in accordance with the equation (2) and provides superior focusing error signal. The direction of the astigmatism can be adjusted by rotating the flat plate 12 about the optical axis 13. The magnitude of the astigmatism can be changed by the inclination .theta., the thickness t or the reflection index n of the parallel flat plate.
However, in the conventional device, a mechanism supporting the flat plate was complicated, since the flat plate for reducing the astigmatism was mounted movable to adjust rotation and inclination .theta.. In addition, when the optical system including the flat plate was to be accommodated in a housing, the mechanism for adjusting the rotation and inclination of the flat plate must be provided in the housing, which mechanism was complicated.